445 



Abstract — Examination of hard parts 

 recovered from scats (feces) is cur- 

 rently the most common method for 

 determining the diet of pinnipeds. 

 However, large or sharp prey remains 

 may be spewed (regurgitated) biasing 

 prey composition and size estimations 

 in diet studies based on scats. Percent 

 frequency of occurrence (F0%) and 

 age or size of selected prey remains 

 recovered from northern fur seal 

 (Callorhinus ursinus) scat (/i = 3444) 

 and spew samples (n=267) collected 

 from rookeries on St. George Island 

 and St. Paul Island, Alaska, between 

 1990 and 2000 were compared to 

 determine if a bias in prey compo- 

 sition and age or size estimations 

 existed between scats and spews. 

 Overall prey composition was simi- 

 lar between sample type and location, 

 but the relative F0% of primary prey 

 (a5%) varied by sample type and loca- 

 tion. Age or size estimates of wall- 

 eye pollock iTheragra chalcograinma) 

 and of two species of gonatid squids 

 (Gonatopsis borealis and Berryteuthis 

 magister) were significantly larger in 

 spews than in scats. Observed differ- 

 ences in F0% and estimated age or 

 size of prey species whose remains 

 were found in scats and spews likely 

 result from size-selective digestion 

 of prey remains. Scats were biased 

 toward smaller prey remains, whereas 

 spews were biased toward larger prey 

 remains and cephalopod beaks. The 

 percent overlap between age classes 

 of walleye pollock caught by the 

 commercial trawl fishery and age 

 classes of walleye pollock consumed 

 by northern fur seals varied notice- 

 ably between sample types for both 

 islands (scats: St. George = 15. 59c: St. 

 Paul=4.19f ; spews: St. George = 94.6'X; 

 St. Paul = 89.6'7f ). These results dem- 

 onstrate that the inclusion of multiple 

 sampling methods allows for a more 

 accurate assessment of northern fur 

 seal prey occurrence and prey age 

 and size. 



Application of two methods for determining diet 

 of northern fur seals (Callorhinus ursinus) 



Carolyn J. Gudmundson 



Tonya K. Zeppelin 



Rolf R. Ream 



National Marine Mammal Laboratory 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NCAA 



7600 Sand Point Way N.E 



Seattle, Washington 98115 



E-mail address (for C J Gudmundson) Carolyn Jenkinswinoaa.gov 



Manuscript submitted 26 October 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 12 October 2005 by the Scientific Editor. 



Fish. Bull. 104:445-455(2006). 



Pinniped diet analysis involves the 

 identification of prey remains recov- 

 ered from a variety of sample types. 

 Prey remains are obtained from 

 stomach lavage and enema proce- 

 dures from feces (scats) and regur- 

 gitations (spews), and from stomachs 

 and gastrointestinal tracts. Histori- 

 cally, northern fur seal {Callorhinus 

 ursinus) diet studies have relied upon 

 analysis of stomach and intestinal 

 contents collected from scientific takes 

 or harvested animals (Wilke and 

 Kenyon, 1957; Kajimura, 1984; Bigg 

 and Fawcett, 1985; Sinclair et al., 

 1994) and, more recently, scat samples 

 (Sinclair et al., 1996; Antonelis et al., 

 1997). Because a thorough analysis 

 of gastrointestinal contents generally 

 requires the sacrifice of an animal, 

 intact gastrointestinal specimens are 

 no longer typically used in diet analy- 

 ses. Lavage and enema procedures are 

 also not widely used to describe the 

 diet of the population because sample 

 sizes are often small and animals 

 may require chemical immobilization, 

 thereby increasing the risk of injury or 

 fatality (Antonelis et al., 1987; Harvey 

 and Antonelis, 1994). 



Presently, scats are the most com- 

 monly used sample type in pinniped 

 diet analyses because they are easy 

 to collect, abundant, and noninvasive 

 (Harvey, 1989; Hammond and Prime, 

 1990). However, many studies have 

 demonstrated the potential biases as- 

 sociated with this sampling method 

 (Jobling and Breiby, 1986; Pierce and 

 Boyle, 1991; Bowen, 2000), including 

 the accumulation of cephalopod beaks 



in the stomach (Bigg and Fawcett, 

 1985; Gales et al., 1993; Harvey and 

 Antonelis, 1994) and underestimation 

 of size and frequency of occurrence 

 of some prey species (Bigg and Faw- 

 cett, 1985; Harvey, 1989; Tollit et al., 

 1997). Captive feeding studies of nu- 

 merous pinniped species have shown 

 that factors such as species, sex, in- 

 dividual activity level, stomach size, 

 gut length, prey digestibility, feeding 

 regime, and meal size affect the de- 

 gree of erosion and recovery of prey 

 remains in scats (Harvey and Antone- 

 lis, 1994; Tollit et al., 1997; Marcus 

 et al., 1998; Bowen, 2000; Orr and 

 Harvey, 2001). Although studies have 

 been conducted to account for these 

 biases through the use of correction 

 factors (Sinclair et al., 1994; Antone- 

 lis et al., 1997; Tollit et al., 1997), 

 the different retention and digestive 

 rates of prey remains in the stomach 

 continue to be a leading criticism for 

 diet studies in which scat samples 

 alone are used. 



Spews have been analyzed for sev- 

 eral species of pinnipeds (Gales et al., 

 1993; Harvey and Antonelis, 1994; 

 Kiyota et al., 1999; Lowry and Car- 

 retta, 1999; Kirkman et al., 2000). 

 Spews contain prey remains such as 

 fish bones, otoliths, and cephalopod 

 beaks. Often these prey remains are 

 too large to pass through the pyloric 

 sphincter and are, therefore, regur- 

 gitated from the stomach (Bigg and 

 Fawcett, 1985; Jobling and Breiby, 

 1986). Although spewings may be 

 found in areas where pinnipeds come 

 ashore, they are often less abundant 



